The present invention relates to drive circuitry for electronic imaging devices, for example, electrographic printers of the type wherein a print cartridge generates charge carriers and directs them at a recording or imaging member by the selective activation of electrodes of an electrode array. It is particularly directed to such printers wherein one set of electrodes is activated with a voltage to function as a generator of charge carriers, and a second set of electrodes are activated to extract or accelerate the charge carriers toward a latent imaging member.
Print cartridges of this type are described in U.S. Pat. No. 4,160,257, U.S. Pat. No. 4,628,227, and others. In the print cartridges described more particularly in the aforesaid patents, a set of electrodes are activated with an RF frequency signal of about several thousand volts amplitude to create a localized corona or glow discharge region. Lesser control voltages synchronized with the RF actuation are applied to one or more control electrodes located at or near the discharge region to gate positive or negative charge carriers from the region, and the print cartridge is biased with respect to an imaging member to maintain an accelerating field therebetween, thus depositing latent image charge dots on the imaging member as that member moves past the print cartridge.
In printing devices using this type of print cartridge, the RF driven corona generation lines extend along the width of the print cartridge, spanning many of the control electrodes, which cross them at an angle. One commercial embodiment, by way of example, has twenty parallel RF lines, which are crossed by one hundred twenty eight oblique control electrodes, known as finger electrodes. During the time when one RF line is activated, by a burst of approximately five to ten cycles of a several MHz drive signal with a peak to peak amplitude of approximately 2700 volts, those finger electrodes which cross the RF line at the desired dot locations are activated to deposit charge dots.
In conventional drive circuitry for such systems, the RF drive lines are actuated in a fixed sequence independently of the image being printed, while during any given RF line actuation, the number of finger electrodes which are actuated varies in accordance with the pattern being printed. After a slight delay for the RF voltage to ramp up, the designated finger electrodes are turned on to cause charge carriers to pass from the print cartridge and accelerate toward the drum, belt or other latent imaging member. Specifically, during their "OFF" cycle, each finger is back biased by several hundred volts with respect to the screen voltage; during their "ON" cycle, the finger voltage is switched to approximately the same potential as the screen.
In the original printers of this type, the individual finger electrodes were switched on for a fixed interval substantially co-extensive with the RF corona generation burst. Such operation produces a fixed amount of charge per actuation. More recently, in U.S. Pat. No. 4,841,313 of Nathan K. Weiner, constructions with a finger pulse of varying duration have been proposed. This operation varies the amount of charge deposited at each dot.
In print cartridges of the aforesaid type, the positive or negative half-cycles of individual RF waves applied to the RF electrodes generate charge carriers, and thus each one defines a basic quantum of charge which may be deposited as a latent image dot. In order to achieve a reasonable range of grey scale charge values using the control of U.S. Pat. No. 4,841,313 it is therefore necessary to provide a larger number of cycles in each burst of the RF line drive signal. This requires the use of a higher RF frequency, or a greater interval of time, for printing each dot, thus entailing trade-offs either in terms of circuit cost or of machine operating speed.
Another approach to producing grey-scale charge images is described in U.S. Pat. No. 4,992,807 of inventor Christopher W. Thomson. Printing apparatus as described in that patent operates by driving an RF electrode with a burst of RF energy, and developing a varying extraction potential synchronized with the RF burst. A finger electrode is then activated for one or more intervals, in phased relation to the variation of the potential, to gate the desired amount of charge from the print cartridge. The finger electrode may be activated with a pulse width modulated pulse and the extraction potential varied monotonically, or the finger electrode may be activated with one or more separate short pulses, each synchronized with a particular portion of the extraction potential curve. As further described in that patent, the finger actuating signal is shifted in width and offset in time so that charge carriers released by the RF excursions occurring during the RF actuation interval are subject to a several different extraction potentials. In any of these constructions, the varying extraction potential modulates the total amount of charge delivered by the print cartridge to form a latent image dot of controlled charge level.
In the aforesaid constructions, it is desirable to control the ON and OFF printing states with accuracy. When the control involves switching the bias potential of finger electrodes, the number and density of the electrodes make the implementation of suitable drive circuitry relatively complex and expensive.
In one presently available finger driver circuit, each finger is driven ON by a high voltage transistor, and when the transistor is not active, the finger potential is passively pulled up through a biasing resistor. That circuit has a relatively high component count, and suffers from a slow rise time not suitable for microsecond resolution of charge gating. Thus, it could not be applied to gate out an arbitrary or changing number of cycles of the RF actuation pulse-generated charge carriers.
In other embodiments of a finger drive circuit, component count can be reduced by replacing the discrete transistors by a small number of integrated circuits, such as the Supertex HV55 display driver chip, which contain shift registers, latches and output transistors with suitable voltage ratings. The positive terminal of the finger driver power supply may be directly connected to the back bias OFF potential, but even so, the pull-up function would have to be performed by an active device, e.g., a switching transistor, to achieve a fast rise time.
In view of the large number of finger electrodes, it is desirable to reduce component count or simplify the circuitry used in driving each finger, while still achieving a responsive and reliable voltage setting. It is further desirable to provide diagnostic functions.